1. Field of the Invention
The present invention relates to an information processing apparatus for converting input color data indicating an achromatic color into output color data indicating a simple black color.
2. Description of the Related Art
FIG. 1 schematically illustrates color matching between different devices.
Input data, such as RGB data or CMYK data, is converted into XYZ data in a device-independent color space by using an input profile. Colors outside the color reproduction range of an output device cannot be reproduced by the output device. Accordingly, color-space compression is performed on the input data that has been converted into device-independent color-space data so that the input colors can be contained within the color reproduction range. After color-space compression, the input data is converted from the device-independent color space into CMYK data in a device-dependent color space.
In color matching, a reference white point and the ambient light are fixed. For example, in the profiles defined by the International Color Consortium (ICC), the Profile Connection Space (PCS) for connecting the profiles is set to D50-standard XYZ values and Lab values.
However, when the same sample (for example, an image) is observed under different light sources, the XYZ values obviously become different between the different light sources. To predict XYZ values under different light sources, there are conversion methods such as (1) ratio conversion, (2) Von Kries conversion, and (3) prediction equations using color perception models.
In the ratio conversion method, a ratio conversion W2/W1 is performed for converting the XYZ values under a reference white point W1 into XYZ values under a reference white point W2. When this conversion method is applied to the Lab uniform color space, the Lab values under W1 and the Lab values under W2 coincide with each other. For example, when the XYZ values of a sample under W1 (Xw1, Yw1 Zw1) are (X1, Y1, Z1), and when the XYZ values of a sample under W2 (Xw2, Yw2, Zw2) are (X2, Y2, Z2), the following relationships are obtained by the ratio conversion.X2=(Xw2/Xw1)·X1Y2=(Yw2/Yw1)·Y1Z2=(Zw2/Zw1)·Z1  (1)
In the Von Kries conversion method, the ratio conversion W2′/W1′ in a human color perception space PQR is performed for converting the XYZ values under W1 into XYZ values under W2. When this conversion method is applied to the Lab uniform space, the Lab values under W2 and the Lab values under W1 do not coincide with each other. For example, when the XYZ values of a sample under W1 (Xw1, Yw1, Zw1) are (X1, Y1, Z1), and when the XYZ values of a sample under W2 (Xw2, Yw2, Zw2) are (X2, Y2, Z2), the following relationships are obtained by the Von Kries conversion.
                                                                        [                                                                                                                                                          X2                                                                                                                                Y2                                                                                                                                                                          Z2                                                                      ]                            =                                                                                          [                                              M                                                  -                          1                                                                    ]                                        ⁡                                          [                                                                                                                                                                  P                                2                                                            /                                                              P                                1                                                                                                                                          0                                                                                0                                                                                                                                0                                                                                                                                              Q                                2                                                            /                                                              Q                                1                                                                                                                                          0                                                                                                                                0                                                                                0                                                                                                                                              R                                2                                                            /                                                              R                                1                                                                                                                                                        ]                                                        ⁡                                      [                    M                    ]                                                  ⁡                                  [                                                                                                                                                                        X1                                                                                                                                          Y1                                                                                                                                                                                          Z1                                                                              ]                                                                                                        where              ,                                                                                          [                                                                                                                                                                                        P                              1                                                                                                                                                                                          Q                              1                                                                                                                                                                                                                              R                        1                                                                                            ]                            =                                                [                  M                  ]                                ⁡                                  [                                                                                                                                                                                                        X                                w1                                                                                                                                                                                                        Y                                w1                                                                                                                                                                                                                                                  Z                          w1                                                                                                      ]                                                                                                                        [                                                                                                                                                                                        P                              2                                                                                                                                                                                          Q                              2                                                                                                                                                                                                                              R                        2                                                                                            ]                            =                                                [                  M                  ]                                ⁡                                  [                                                                                                                                                                                                        X                                w2                                                                                                                                                                                                        Y                                w2                                                                                                                                                                                                                                                  Z                          w2                                                                                                      ]                                                                                                                        [                M                ]                            =                              [                                                                            0.40024                                                              0.70760                                                                                      -                        0.08081                                                                                                                                                -                        0.22630                                                                                    1.16532                                                              0.04570                                                                                                  0                                                              0                                                              0.91822                                                                      ]                                                                                                        [                                  M                                      -                    1                                                  ]                            =                              [                                                                            1.85995                                                                                      -                        1.12939                                                                                    0.21990                                                                                                  0.36119                                                              0.63881                                                              0                                                                                                  0                                                              0                                                              1.08906                                                                      ]                                                                        (        2        )            
According to the prediction equation using color perception models, conversion is performed by using a human color perception space QMH (or JCH), such as CIECAM97s, for converting the XYZ values under viewing condition VC1 (including W1) into XYZ values under viewing condition VC2 (including W2). In QMH, Q indicates the brightness, M represents the colorfulness, and H designates the hue-quadrature or hue-angle, respectively. In JCH, J indicates the lightness, C represents the chroma, and H designates the hue-quadrature or hue-angle. When this conversion method is applied to the Lab uniform color space, the Lab values under W1 and the Lab values under W2 do not coincide with each other, as in the Von Kries conversion method. For example, when the XYZ values of a sample under W1 (Xw1, Yw1, Zw1) are (X1, Y1, Z1), and when the XYZ values of a sample under W2 (Xw2, Yw2, Zw2) are (X2, Y2, Z2), the following conversion is performed by using the color perception models.X1, Y1, Z1)→[CIECAM97s forward conversion]→(Q, M, H) or (J. C, H)→[CIECAM97s inverse conversion]→(X2, Y2, Z2)  (3)
An example of color matching implemented under different viewing conditions by using color perception models is shown in FIG. 2.
As shown in FIGS. 1 and 2, color matching between different devices can be achieved by using device-independent XYZ values (or Lab values). However, when converting XYZ values into CMYK values, the black-printing (K-printing) generation characteristic must be fixed so as to obtain one K value since a plurality of combinations of CMY values and K values are available for single XYZ values.
If the black-printing generation characteristic is fixed, a combination of CMY value and the K value can be uniquely determined for the XYZ value. However, when converting CMYK values into CMYK values, even if the combination of the input CMY values and K value is changed, such a change cannot be reflected in the resulting output data since the black-printing generation characteristic is fixed. For example, it is now assumed that CMYK values A1′ are output in response to input CMYK values A1. In this case, even if CMYK values A2, which represent the same color as the CMYK values A1, are input by increasing the CMY values and by decreasing the K value, the resulting output data remains the same as the CMYK values A1′ in accordance with the fixed black-printing generation characteristic since the XYZ values are the same.
For the same reason, when a gray color is output in response to input CMYK values, even if the simple K color (0, 0, 0, K) is input, the resulting output data does not become a simple K color (0, 0, 0, K′).
In the field of graphic art, characters in CMYK values are in many cases represented by a simple K color. It is thus desirable that an input simple K color be output as a simple K color.
Additionally, it is desirable that a simple K color be output in response to input RGB values (when R=G=B).